The present invention relates to ballast circuits for use with LEDs (light emitting diodes), and more specifically, to an improved dimmable ballast circuit for LEDs powered by AC (alternating current) finding particular use in aerospace lighting panels.
Ballasts, in their most commonplace form, are used in conjunction with flourescent and other gas discharge light bulbs. A fluorescent lamp ballast is a device used to start and operate a fluorescent lamp and is a vital part of the lighting fixture. It provides the three-step action needed by a fluorescent lamp: controlled energy to heat the electrodes (filaments); the right voltage to start the arc; and impedance to limit the current to the proper value. To give optimal lighting performance, the ballast must supply the specific electrical values established by the fluorescent lamp manufacturer.
A further function of the ballast is to prevent the destruction of the lamp. Unlike an incandescent bulb or LED, if a fluorescent lamp were connected directly to AC power, it probably would not light. If it did light, the increase in current would soon destroy the lamp, since once the arc begins, the impedance of the lamp drops to a low value. Therefore, the ballast must provide additional impedance to limit current to the proper value. When improper electrical values are supplied by the ballast, the light output and life of the lamp may be greatly reduced.
The first common type of ballast is the electromagnetic ballast. Electromagnetic ballasts employ an inductor and a power capacitor. The inductor consists of a core of steel laminations surrounded by one, two or more copper or aluminum coils. The inductor provides the conditions for starting and controlling the current flow to the fluorescent lamp. Prior to the 1980""s, the material chosen for the core and coils was usually driven by economics to minimize the ballast cost while meeting performance requirements. These ballasts are usually referred to as standard or conventional magnetic ballasts. Many of these ballasts are still in service today. During the 1980""s more efficient designs started to gain some popularity. These designs, commonly referred to as energy efficient magnetic, are optimized for maximum efficiency. Since 1990, only energy efficient magnetic ballasts have met the U.S. efficiency regulations for most popular lamp configurations. This type of ballast is often enclosed in a metal case filled with an asphaltic compound that helps dissipate heat and control ballast sound.
Hybrid ballasts are an alternate design which start like rapid start ballasts but reduce or remove the electrode heating after the lamp is in full operation. Such ballasts are sometimes also referred to as cathode cutout ballasts. Some lamps have slightly reduced longevity when operated with these modified rapid start ballasts.
Employing more advanced technology, electronic ballasts operate lamps at high frequencies, using semiconductor components to change the frequency of the incoming AC power in combination with small inductive and/or capacitive components to provide the starting and regulating function. Electromagnetic ballasts operate the lamps at line frequency, usually 60 Hertz (Hz). Electronic ballasts convert the line frequency to frequencies between 20 and 60 kilohertz (kHz). It is well known in the art that low pressure arcs such those in fluorescent lamps are more efficient when operated at high frequencies. For many popular lamps, this increase in efficiency is about 10 percent. Furthermore, electronic construction weighs less than coil and core magnetic construction, allowing easier handling during installation, lower structural stress on ceiling supports and lower shipping costs.
Prior art avionics displays have utilized the aforementioned flourescent lighting and ballast technologies. Kalmanash U.S. Pat. No. 5,211,463 teaches a backlighting system for aircraft displays comprising distinct day viewing and night viewing configurations. The day viewing configuration may comprise a standard flourescent lamp and ballast for full color display while the night lighting system may comprise an additional lamp with appropriate infrared filtering for night vision compatibility. The system is not dimmable and is complex because of the additional ballast circuitry necessitated by the second lamp. Furthermore, because of the plurality of bulbs and ballasts, the cost of the system is high.
Fischer U.S. Pat. No. 5,296,783 discloses another flourescent lamp for use in aircraft displays. A flourescent lamp having dual filaments and dimming capabilities is taught. However, as is well known in the ballast art, a flourescent lamp may only be dimmed approximately 30 to 50 percent, and often at the cost of lamp efficiency and longevity. This is because at lower supply voltages the flourescent filament cannot heat to a temperature sufficient for thermionic emission and thus is undergoing thermal stress without producing light.
Alternatively, aircraft instrumentation lighting also widely utilizes incandescent lamps. Such lighting often produces high brightness and sunlight readability, both of which are very desirable features because aircraft are often at altitudes of 40,000 feet in daylight sun. However, since the lighting is produced by the heating of a lamp filament whose radiant emissions are primarily heat, reliability decreases. This ultimately leads to high failure rates and overall high maintenance rates throughout the service life of the aircraft.
LEDs, however, eliminate the problems inherent in incandescent lighting, i.e., high power consumption, high heat generation and high touch temperature (as per MIL-STD-1472). LEDs produce a brightness equivalent to incandescent bulbs while using only a third as much power. The reliability and high maintenance problems are eliminated by the use of highly reliable, long life LEDs. For example, with ballasts operating at 28 volts DC, tests done in accordance with MIL-HDBK-217F, Notice 2, reveal that an LED will last on average 85,000 hours whereas a typical incandescent lamp will last only 2,800 hours, i.e., the LEDs last over 30 times as long.
In view of the foregoing, the present invention addresses the shortcomings of the prior art by providing a dimmable LED ballast having high brightness, heightened reliability and efficiency, all while reducing cost.
The present invention relates to ballasts for LEDs finding particular use in aircraft lighting panels.
In a first embodiment, AC power is used to illuminate anti-parallel connected LEDs. A ballast inductor is used to limit current, while a capacitor is shunted across the input terminals to ensure a unity input power factor. This circuit is further modified by the addition of a transformer, thereby adapting the circuit for use with any input AC voltage. In this case, the shunt capacitor compensates for both the inductor and the magnetizing inductance of the transformer primary. Also, a circuit for use with a multitude of LEDs is taught that ensures equal brightness for all connected LEDs.
All of the foregoing circuits have a linear transfer characteristic, i.e., the LED current varies linearly with input AC voltage. This behavior is unsuitable because it does not emulate that of an incandescent bulb, and therefore these circuits are not compatible with conventional AC light dimmers. Thus, these circuits cannot readily take the place of an incandescent bulb in a pre-existing lighting panel. Therefore, alternate ballast embodiments are taught which use shunted diodes to short out one or several series connected inductors to steepen the slope of the transfer function, thereby more closely matching the behavior of incandescent bulbs.
Lastly, an embodiment is taught that accepts DC input and uses an oscillator circuit to convert the DC input to AC. Any of the above taught embodiments may be used along with the oscillator circuit.
Thus, it is an object of this invention to provide an LED ballast operable with a variety of AC voltages.
Further, it is an object of this invention to provide a dimmable LED ballast operable with a variety of AC voltages.
It is an additional object of this invention to provide a dimmable LED ballast capable of driving a multitude of LEDs with the same current.
It is yet another object of this invention to provide an AC powered dimmable LED ballast having a non-linear transfer characteristic.
Additionally, it is an object of this invention to provide a dimmable LED ballast having an oscillator to convert DC to AC.
Further, it is an object of this invention to provide a dimmable LED ballast that is a direct replacement for incandescent lighting.
It is yet another object of this invention to provide a dimmable LED ballast that does not produce any electrical interference.
It is a further object of this invention to provide a dimmable LED lighting system finding particular use in aerospace applications that is night vision compatible.
Furthermore, it is an object of this invention to provide a dimmable LED ballast using only passive components.